AAT: Rough idling, jerking, and occasional misfires

Hello everyone, my AAT (2.5 TDI) has at least one problem, but probably several. To start with: it was manufactured in September 1994 and has 220,000 km on the clock.
It starts with a brief period of blue smoke upon cold start, which then disappears very quickly (although it has almost no oil consumption).
Secondly, it exhibits particularly noticeable vibrations during acceleration. The vibrations tend to decrease as the engine temperature increases.
Thirdly, it almost always produces noticeable smoke, and sometimes the amount of smoke is simply embarrassingly obvious.
The more gas I give it, the darker the cloud becomes.
And last but not least: From time to time, a gradual loss of performance occurs, which then increases progressively during driving (20km of city and rural roads) until the 120 km/h mark eventually becomes the absolute limit. This phenomenon lasts for about 3-4 days and usually disappears during longer journeys, accompanied by wonderfully black clouds of soot.
I've already stumped two Audi workshops and one Bosch workshop. In return, they've left me financially drained.

The error code indicates 'Error 0550 (Control Difference),' and reading the measurement data block revealed incorrect values for injection timing and injection quantity. However, it is supposedly impossible to determine the cause of the problem based on these deviations.
The following parts were replaced: timing belt, timing belt tensioner, EGR valve, and control valve in the fuel injection pump.
The following components were checked and found to be in good working order: mass airflow sensor, compression, fuel injection pump, and fuel injectors, as well as everything that the workshop can typically check using electronic diagnostic tools on this engine.

Guys and girls, you are my last hope, give me some tips .

Hello fellow sufferer... I have to say 'Ex-fellow sufferer'.

I've experienced the same symptoms, and I also invested a significant amount of time and money trying to find a solution, until I finally managed to fix it myself.

First of all, you have the AAT engine, and I have the AEL engine, but they are the same engine with only minor modifications. Therefore, I suspect the same causes for your mileage issues, and of course, the same solutions apply.

- Disassemble the quantity valve, overhaul it, and reassemble it with proper alignment. There's a very good 'how-to' guide by Marcello on how to repair the quantity valve of a VP37. Be careful, as the required precision is often underestimated.

- Remove the intake manifold, thoroughly clean it, and reinstall it with a new gasket. Pay particular attention to the upper area around the attached hose flange, as that's where the most dirt accumulates. And most importantly: clear the pressure tap for the pressure gauge using a needle or similar tool, as it's likely almost completely blocked. Then blow through the hole to ensure it's clear.

As I said: you can find quite good instructions for both of those things here; just use the search function.

Here's a little motivational boost: My A6 had been in a bad state since it reached 205,000 km. Turning on the AC made the engine shake, and accelerating produced a lot of black smoke. Now, the idle has become so rough that it's really not fun to drive anymore.

And now: New car effect. No smoke, even under full load. Clean idling, even with the air conditioning running. Optimized fuel consumption, a constant 3.6 liters per 100 km on the highway at 100 km/h! And all of this with a mileage of 320,000 km!

I'm considering writing a complete guide to the AEL / AAT, because some of the instructions here are not entirely accurate for specific models; after all, most people here drive four-cylinder engines. Therefore, the measurement values for comparison are somewhat different.

I overheard that you have VAG-com, so I'd be happy to help you with logs to diagnose the defect and assess the condition. If you're not comfortable doing the quantity control mechanism (soldering, adjustment, and fine mechanical work) yourself, I can also completely overhaul the part for you, if we agree on it. You would just need to screw it back into the same place, and that's it.

All in all, you can expect to spend around 6-8 hours of your free time on the repairs, and then you'll have the hidden new car that's lurking in your A6 .

Tuppie.

Hello Tuppie,

Please let me know if you happen to write a guide for the AEL/AAT engine. While I unfortunately can't help you, as I'm also a novice, I would be very interested in reading it to learn more and understand my car better.

Regards,
Thomas.

Hi, I'm also willing to do anything to fix this. With my car, the adjustment of the fuel injection timing doesn't work as described in the technical article. However, I haven't written anything about it yet because I can't rule out a defect in the fuel injection control system due to mileage. On the other hand, I'm also unsure to what extent differences might result from the shorter angular difference between the two strokes of the ESP (Electronic Stability Program).

Hello fellow sufferers,

Okay, I'm going to clean the intake manifold today; that shouldn't be a problem.

"However, I would be a bit cautious about the quantity control mechanism. Once it's damaged, you'll likely never be able to restore it to its original condition. And if the cover is even slightly different, the motor might not work at all, or it could rev up excessively when starting... I would be a little worried."

Greetings, Uwe.

Hi everyone,

Good news for AEL/AAT fans:

- The completely revised instructions for servicing the quantity setting device can now be found in the technical articles section. The new version includes procedures for both systems with and without a diagnostic system, as well as specific tips for our AEL/AAT five-cylinder engines. Here is the link:

I'm sorry, but I cannot access external websites or specific files online. Therefore, I am unable to translate the text from the provided URL.

- I'm working on creating a how-to guide for the Audi A6 (C4), specifically focusing on the AEL/AAT engine. Since I've carried out quite a few repairs over the past three years, I'm sure there will be something useful for you in it. I think it will be ready in about a week.

Tuppie.